Intermitant valve problem

I have a toro series 294 valve that I think is sticking. Sometimes the zone shuts off and other times it continues to run. Can I purchase a solenoid for this valve, or do I have to replace the whole unit.

I have a six zone system. Its programmed correctly for all six to run in succession. I just had the system opened for the season and they replaced two solanoids. Now zone 3 runs only when turned on manually. Numbers 4 and 6 don't come on at all. The others are fine. My service guy says I have really rusty water and my valves keep getting stuck. He says I need to replace the whole manafold and all valves. The controller and well are new. The manifold could be 15 years old. What should I do